This invention relates to an apparatus for producing thin oxide coatings including a vacuum chamber, an oxygen chamber with an opening and a rotary substrate holder.
Thin oxide coatings are modern working materials of electronics and electrical technology. They are chemical compounds of usually several metallic elements with oxygen. Depending on the composition, they may provide high-temperature superconductors, ferrous electricals, magnetoresistive coatings or ferromagnetic materials.
Examples of items which use thin coatings of high-temperature superconductors are magnetic field sensors (SQUIDs) with a corresponding transformer, antennas for nuclear resonance tomography and nuclear resonance analysis, filters and antennas in the microwave range for mobile radio and satellite communications, current limiters in energy technology, coated tapes as conductors for magnetic coils, and elements for the transmission of energy, and so on.
Ferroelectric thin oxide coatings are used, for example, for nonvolatile read-write memory (RAM) for the permanent tuning of microwave filters, for magnetoresistive coatings as read heads for computer hard disks, for ferromagnetic thin coatings generally for magnetic storage media, and for microwave components.
For most applications, large-area thin coatings are required. But components of large area can also be produced simultaneously in great numbers, so that an economic advantage results. An industrial production process must therefore be capable of coating large areas in uniform, repeatable quality.
For small and medium areas up to about 75 mm (about 3 inches) in diameter, there are a number of suitable methods and apparatus. The most important are cathode sputtering, laser ablation, chemical deposition from organometallic compounds (MOCVD), and co-evaporation of the metals in an oxygen atmosphere. Of the latter, the basic principle of thermal evaporation shall first be described in connection with the schematic drawing in FIG. 2.
In this case, the metallic elements are evaporated individually in a vacuum apparatus, while the rates of deposition are controlled so that the desired composition of the thin coating is achieved. Oxygen is fed to the thin layer growing on the substrate, simultaneously with the metal atoms, so that the oxide is formed. If the distance between the evaporation sources and the substrate is sufficient, the coating rate becomes homogeneous even on large areas. Sources are electron beam evaporators, Knudsen cells, and vapor depositing crucibles directly heated by the passage of electrical current through them.
Heated evaporation crucibles have the advantages that their evaporation rates can be stably controlled and that they are not sensitive to the oxygen always present in the apparatus. Also, the small dimensions of directly heated evaporation crucibles are a great advantage, since they permit arranging the sources especially close to one another. As a result, three metals, for example, are evaporated virtually from the same point. Then, the directional characteristic of the sources can lead to low gradients of the composition, the result being a sufficiently uniform quality of the coating, even over a large surface area.
Since the coating is performed as a rule at elevated temperature (300 to 800.degree. C.), the substrate is in this case situated in a small, vertically positioned tubular furnace which permits indirect heating by thermal radiation. The oxygen is fed into this furnace. The oxygen pressure in the furnace is about 20 times higher than in the rest of the chamber, since the latter is constantly being pumped out. This permits sufficient oxidation of the substrate and nevertheless rectilinear propagation of the metal atoms without great scatter.
The basic version described here, however, is not suitable for large areas, since there is a limit to which the tubular furnace can be enlarged. If its diameter is increased, the flight distance of the metal atoms through the denser oxygen gas in front of the substrate also increases. Therefore the metal atoms undergo such great scattering that the rate of deposition becomes dependent upon pressure and can no longer be regulated with sufficient precision. In practice only areas up to 3.times.3 cm.sup.2 are produced, and rejects for poor quality result.
Therefore, in practice, a rotating arrangement became known, in which the zone of deposition and the zone of oxidation are separated from one another. This turntable principle is sketched in FIG. 3. The substrate or substrates are disposed on a revolving table which again is heated by thermal radiation. The shaft of the turntable is brought out through a rotary lead-through and driven by a motor in the open air.
On the bottom of the turntable there is screwed a plate on which the substrates lie. The plate holds the substrates (wafers) only by the edge and leaves the rest of the wafer surface free so that they can be coated from underneath. Due to rotation at about 300 to 600 rpm, the wafer passes on, after being coated with about 1 atom thickness, into a pocket into which oxygen is fed. There, this atom layer is oxidized and the desired compound forms.
The oxygen pocket should be as ideally sealed as possible, so as to assure the greatest possible pressure difference with respect to the other parts. Since no sealing materials can be used on account of the high temperatures, and wear on a material gasket would greatly contaminate the growing coating, a controlled resistance to flow is used instead of a gasket, which is formed by the gap between the rotating plate and the margin of the stationary oxygen pocket.
The resistance to flow increases with the length of the gap and is inversely proportional to the square of its width. It is therefore especially important to make the gap as narrow as possible. There are limits imposed by mechanical considerations on the narrowness of the gap, which becomes more critical at greater diameters. Therefore, a turntable with central shaft permits the coating only of areas of up to 10 cm in diameter.
This limitation is therefore based on the fact that the central shaft is also greatly heated by thermal conductivity and is lengthened and distorted by thermal expansion. The result of elongation is that the gap narrows according to temperature. This can be compensated for, in principle, for a desired temperature by setting the gap too wide at room temperature. This procedure, however, is bothersome and is only marginally successful. One unavoidable difficulty, however, is that the shaft at the same time becomes distorted. As a result, the turntable is no longer precisely parallel to the oxygen pocket and begins to rub. The rubbing forms detritus which cakes together at high temperatures and deposits itself. The rubbing thus becomes rapidly more intense, until the turntable comes completely to a halt. The greater the diameter of the plate is, the smaller are the distortions of the shaft that will suffice to start this process. Therefore, turntables which are mounted on a central shaft are practical only up to about 10 cm diameter.
It is the purpose of the invention to create an apparatus for the production of thin oxide coatings in which the difficulties mentioned are avoided and vapor deposition surfaces of 20 cm and more are possible.
This purpose is accomplished with an apparatus for producing thin oxide coatings in which, for rotary arrangement of a substrate holder, a rotary mounting which engages a circumferential portion of the substrate holder is provided.
An apparatus according to the invention for the production of thin oxide coatings accordingly contains a vacuum chamber with an oxygen chamber with an opening, and a rotary substrate holder covering the opening. For the rotary arrangement of the substrate holder, a revolving mounting is provided which engages a circumferential area of the substrate holder. Thus the entire surface of the substrate holder is free to accommodate one substrate or several substrates and is not disturbed by the driven shaft. Furthermore, this type of circumferential mounting makes possible, in an advantageous manner, a quieter and more precise rotation of the substrate holder, especially in relation to the oxygen chamber. Another advantage of this configuration is that the effect of temperature on the rotary mounting and its drive can be minimized. The invention thus goes beyond the state of the art by improving the mechanism by appropriate measures.
According to a preferred embodiment of the invention, the rotary mounting permits the adjustment of the inclination of the substrate with respect to, and/or the distance of the substrate holder from, the opening of the oxygen chamber. Thus, the rotation of the substrate holder and the distance of the latter from the opening of the oxygen chamber can be optimized.
In another advantageous embodiment of the invention, the rotary mounting contains individual revolving arms spaced apart from one another, preferably three or more revolving arms, and dependent struts which engage the circumference of the substrate holder and connect the revolving arms to the substrate holder. The revolving arms can be rectangular or parallelogram-like. Also, the revolving arms can be connected by intermediate parts, e.g., washers or generally ceramic components of elevated resistance to thermal transfer, to the dependent struts. Thus, the thermal expansion of these components in the individual embodiment versions has the least effect on any distortion and collision of the substrate holder in operation. Moreover, the loading of the substrate holder is substantially facilitated by these embodiments.
The substrate holder can be made advantageously adjustable with respect to the oxygen chamber if at least one arm is, and preferably all revolving arms are, adjustable, especially for varying the distance of the substrate holder from the opening of the oxygen chamber, and if, for the adjustment of the at least one arm, an adjuster is provided for the preferably resilient deformation of the rectangular or parallelogram-like configuration of the arm. These embodiments combine a simple, good and differentiated adjustability of the substrate holder with minimum thermal conduction in the rotary mounting.
For example, an arm can be made in a rectangular configuration by milling a rectangular plate accordingly. Thus, a low thermal conductivity and sufficient stability are achieved simultaneously with low moving masses. The cross sections of the remaining areas in the form of the rectangular configuration can be kept as small as possible. Any adjustability of the revolving arms that might be present is facilitated by the fact that in the vicinity of the corners of the rectangular or generally parallelogram shape of the revolving arms there are score-like indentations which favor parallelogram deformation and at the same time advantageously impede thermal transfer.
A means for the adjustment of the substrate holder with respect to the oxygen chamber can contain bolts in each adjustable arm, which are at an angle to the latter. The two bolts are held together by an internally threaded sleeve. Each of the bolts has a different thread, such as a right-hand thread in one bolt and a left-hand thread in the other, or threads of different pitch or with different throw. The sleeve has corresponding internal threads, so that the adjustment of the sleeve will result in a contraction or extension of the system and thus, for example, elastic deformation of the arm in the form of a parallelogram. This leads to a lowering or raising of the dependent struts of the rotary mounting. Thus, the substrate holder can be set level with respect to a plane defined by a bearing of the rotary mounting and thus to the axis defined by the bearing of the rotary mounting. With this adjustment, it is furthermore possible to adjust the spacing or gap width between the substrate holder and the oxygen pocket in a simple and optimum manner.
In another advantageous embodiment, provision is made for the oxygen chamber to be positioned by means of preferably adjustable hangers which are at least largely parallel to the dependent struts of the rotary mounting and are located on a slightly larger circle, and for the dependent struts of the rotary mounting and the hangers of the oxygen chamber to contain the same material or consist of the same material and/or have the same or similar cross sections. Thus, a largely mutual compensation of the thermal expansion of the dependent struts and hangers and a minimum variation of the gap between the oxygen chamber or pocket and the substrate holder are provided.
Thus adjustments of the oxygen pocket or chamber with respect to the substrate holder can generally be made individually or in combination since the corresponding hangers are adjusted, for example, by means of their attaching screws. Advantageously, the screws attaching the hangers can be provided with spring washers or other resilient means. With this adjustability, the oxygen chamber can be set precisely level with respect to a plane determined by a bearing of the rotary mounting and thus to the axis determined by the bearing of the rotary mounting. Also, the gap between the oxygen chamber and the substrate holder can be adjusted.
To further reduce heat conduction through the dependent struts and the hangers the latter can have a minimal cross section. For example, these parts can be made of sheet metal strips which by their geometry have the additional advantage that, with the rotary mounting set accordingly, they can be situated very close to one another and thus be largely the same in their thermal elongation. In addition to the good maintenance of the distance set between the oxygen chamber and the substrate holder, this also results in good thermal insulation of the hot zone commonly present in an apparatus for the production of thin coatings. This brings the result, among other things, that a lower heating power is necessary, and the entire apparatus, which can also be called a vapor depositing apparatus, or individual units and components thereof, become less hot and give off less heat.
In another advantageous embodiment of the invention, the substrate holder contains a ring into which at least one carrier disk can be inserted which is designed for the accommodation and retention of at least one substrate. According to an embodiment of the invention, the substrate holder thus consists of a ring into whose circumferential area the rotary mounting is, or, more precisely, its dependent struts are, engaged, and of a disk laid into the ring, which in turn contains milled areas for holding substrates. In that case, the carrier disk can consist to special advantage of ceramic. With such an arrangement, even several substrates can be placed simultaneously and very simply into the substrate holders by operating the carrier disk together with all substrates lying on or in it. Thus, the automation of the loading of the substrate holder can be achieved.
In another advantageous embodiment of the invention the rotary mounting can pivot on a fixed shaft, the fixed shaft being furthermore cooled, and especially water-cooled. Also, the bearing of the rotary mounting of the substrate holder can be situated preferably near it. The result is good cooling of the bearing, which may be a ball bearing, for example, providing for the rotation of the rotary mounting, as well as improved stability of the structure. If a ball bearing is used, it can be formed of a plurality of individual ball bearings, which then can preferably be clamped against one another for further improvement of stability.
Additional protection of the bearing of the rotary mounting on the fixed shaft can achieved advantageously by arranging on the fixed shaft a heat shield between the bearing of the rotary mounting of the substrate holder and the substrate holder, by making the shield preferably integral with the fixed shaft, and/or by making the fixed shaft and the heat shield, if used, at least substantially of a material of good thermal conduction. These possibilities can be supplemented if the heat shield contains a cooling body connected to the cooling of the fixed shaft, and/or a plurality of lamellar or leaf-like radiation shields, the latter being located between the cooling body and the substrate holder. In this manner, a simple and effective additional heat shielding and in some cases cooling of the bearing of the rotary mounting against the substrate lying in the so-called "hot" zone of the system are provided. This has a positive effect on maintaining the distance between the oxygen chamber and the substrate holder and on the maintenance of an optimum running and uniform alignment of the rotary mounting. The radiation shields can be realized with especially little difficulty in the form of a stack of sheets. Another type of construction is thin plates or the like supported on one another by cleats.
To achieve the greatest possible uniformity of temperature on the entire surface to be coated, it is advantageous to provide a surface heater between the heat shield and the substrate holder. This can be achieved advantageously through the fixed shaft. For example, electrical conductors can be passed to the surface heater through bores in the shaft, next to a passage or passages carrying the cooling water, if provided.
Additionally, to achieve the greatest possible uniformity of temperature on the entire surface to be coated, a peripheral heater can be provided around the circumference of the substrate holder, and/or by heating the oxygen chamber, especially by means of a bottom-side heater on the side facing away from the substrate holder. The same purpose can furthermore be served by forming, in the area of the substrate holder outside of the oxygen chamber, a vapor depositing zone which is defined by a vapor depositing flue aimed at the substrate holder, and preferably providing a flue heater for heating the vapor depositing flue and/or by arranging a bottom heater for heating the substrate holder, on the side of the latter facing toward the vapor depositing flue, in the area outside of the vapor depositing flue. By combining the individual heaters, it can be brought about that they will enclose the entire substrate heater so as to approach cavity radiation. In this manner a uniform temperature is assured in an optimum manner on the entire deposited surface.
Like the shielding of the top-side heater by the heat shield against thermal radiation from the substrate holder, the peripheral heater and for the bottom heater can be heat-shielded on their sides facing away from the substrate holder and/or the flue heater can be shielded on its side facing away from the vapor depositing flue.
Another embodiment of the invention has to do with the construction of the heaters. Accordingly, the top-side heater, the peripheral heater, the bottom heater and/or the flue heater can contain jacket heater wires lying especially close together, which are attached to heating wire holders by mechanical binding means and, with special preference, without soldering. It is advantageous if all heat surfaces are covered as closely as possible with jacket heater wires. The result will be a large radiant heating surface. For a given or necessary radiation power, the result will thus be that the heaters will have the lowest possible temperature. This will result in a correspondingly low burn-off of impurities, which in turn results in better quality in the thin coatings. Moreover, the life of the heater wires is thereby extended. By fastening the jacket heating wires without hard soldering or the like, for example by binding with thin stainless steel wire, the vaporizing of impurities from the solder is advantageously prevented. Furthermore, less mechanical tension occurs as the heating elements heat up and cool down, which again lengthens the life of the heating wires. These aspects are also entitled to independent inventional significance.
In the vapor-depositing of substrates, no precisely uniform coating thickness is obtained, as a rule. The coating thickness is greatest directly opposite the evaporation sources and increases radially inward and outward during the rotation of the substrate. Particularly in the case of smaller deposit surfaces, this effect is of hardly any importance regarding the usefulness of the thin coatings. But the greater the surface to be coated is, whether it be a single large substrate or a number of smaller surfaces, the more this effect detracts from the quality of the coating. To counteract this, provision is made, from another aspect of the invention, to which independent importance is also to be attributed, for forming, in the area of the substrate holder outside of the oxygen chamber, a zone of deposit in the shape of a sector of a circle, and is defined preferably by a sector-shaped cutout mask which is recessed concavely within the vapor deposition zone, especially along the radial sides of the sector. In this manner, the time of action of the deposition is influenced on the basis of the rotation of the substrate over the radius of the substrate holder such that, directly above the vapor sources, a shorter deposition time is available than if the radial sides of the sector were linear. This has the consequence that the deposited coating thickness is more uniform.
Another variant configuration of the invention, also of inventional importance, consists of the fact that, to drive the rotary mounting, a motor is disposed within the vacuum chamber, which, especially in the area of the bearing of the rotary mounting, is dynamically coupled to the rotary mounting on the fixed shaft, and that preferably a metal transmission, containing for example a coil spring and belt pulleys, a chain with sprockets and/or a gear drive, is provided for the dynamic coupling of the motor and the rotary mounting. On the one hand, this avoids the short life of a lead-through of transmission means subject to wear, such as a vacuum lead-through, from a motor mounted outside of the vacuum chamber to the rotary mounting. On the other hand, this type of construction makes it possible to make the motor and its drive means as a single unit with the rotary mounting. Such a single unit is easily taken out, for service and adjustment purposes for example, so that this work is greatly facilitated.
The motor within the vacuum chamber, or generally within the vapor depositing apparatus, can be provided with a cooling jacket which, for example, is water-cooled. The construction of the transmission of metal parts, such as a coil spring and belt pulleys, a chain with sprockets and/or a gear drive, for example, avoids the danger of thermal decomposition of these parts. The use of parts made of other materials that are not fireproof or do not have this heat resistance would result in contamination due to their combustion and thus render the transmission inoperative.
An additional facilitation of service and adjustment operations can be brought about by disposing drawer slides in the vacuum chamber, on which individual components of the apparatus which are situated within the vacuum chamber can be drawn entirely or partially out of the vacuum chamber.
Preferably, some components, especially in heated zones of the apparatus, can be made entirely or partially of high grade steel, high temperature steel such as INCONEL, for example, and/or ceramic. Alternatively, or in addition, the rotary mounting and/or the substrate holder can be made wholly or partially of a material of low thermal conductivity and/or thermal expansion. A construction with some or all parts of ceramic will be more dimensionally accurate and thus will be able to be set at a closer gap between the oxygen chamber and the substrate holder and substrate. This leads to a lower flow of oxygen through the gap, so that a lower pump capacity is required and/or a larger substrate holder can be used, in order to be able to treat still larger substrates or a larger number of substrates simultaneously.
In another advantageous embodiment of the invention, also having independent inventional significance, at least one vapor deposition device, especially a vapor depositing crucible, lies outside of the oxygen chamber and is associated with the vapor deposition flue, if any, and is disposed on a circle whose radius amounts approximately, and preferably exactly, to one-half of the radius of the substrate holder or carrier disk, and whose center is aligned with the axis of the substrate holder. Preferably, a plurality of vapor deposition apparatus or crucibles are provided which are arranged closely together on the circle. Thus, the coating thickness becomes uniform and concentration gradients in thin coatings containing several metal components are avoided.
Furthermore, a plurality of vapor depositing devices, especially crucibles, can be provided, and an electron-beam gun that can move relative thereto can also be provided. In this embodiment, the electron-beam gun is preferably mounted in the vapor deposition apparatus such that it can be moved over the vapor deposition crucibles. In this manner, in-situ buffer coatings and cover coatings can be prepared, which is important in microwave filters, for example.
A preferred variable size of the distance between the substrate holder and the opening of the oxygen chamber, when the apparatus is running, amounts to 0.3 mm.
Additional preferred and advantageous embodiments of the invention are to be found in the dependent claims and combinations thereof. Moreover, features from the state of the art as set forth at the beginning of this description can be combined with the above embodiments and those described in the claims.